This invention relates to drop-on-demand multi-tone printing.
In particular, a first aspect of the present invention relates to a method of operating a drop-on-demand multi-tone printing apparatus having a series of channels arranged to receive ink from a source, each channel having a respective ink outlet (usually, an appropriately shaped and dimensioned nozzle), each adjacent pair of the channels being separated by a respective dividing wall, and each dividing wall being movable in response to a pulsed electrical signals (for example using the piezo-electric effect) to apply pressure pulses to the ink in the respective pair of the channels, whereby ink can be ejected from the outlets and deposited onto a recording medium.
A problem with operating such an apparatus is that the signal applied to each dividing wall affects both of the channels to either side of that dividing wall. Patent document WO-A-96/10488 describes ways of dealing with this problem.
In one example described in WO-A-96/10488, printing of a line of pixels is divided into three cycles. In the first cycle, the dividing walk to either side of channels numbered, for example, 1, 4, 7, . . . are driven (if ink is to be ejected from them) with a pulsed signal. In the second cycle, the dividing walls to either side of channels numbered 2, 5, 8, . . . are driven (if ink is to be ejected from them) with the pulsed signal. In the third cycle, the dividing war""s to either side of channels numbered 3, 6, 9, . . . are driven (if ink is to be ejected from them) with the pulsed signal. Thus, the pressure pulses developed in the channels which are not included in the current cycle are no greater than xc2xd of those in the channels which are intended to eject ink. The printing apparatus is arranged so that such xc2xd magnitude pulses do not cause ink ejection.
In another example described in WO-A-96/10488, printing of a line of pixels is divided into two cycles. In the first cycle, the dividing walls to either side of channels numbered 1, 5, 9, . . . , for example, are driven (if ink is to be ejected from them) with a first pulsed signal, and the dividing walls to either side of channels 3, 7, 11, . . . are driven (if ink is to be ejected from them) with a second pulsed signal which is xcfx80 radians out-of-phase with the first pulsed signal. If the dividing walls to either side of the odd-numbered channels are all driven, then the resultant pressure pulses applied to the even-numbered channels are zero. If the dividing walls to either side of only some of the odd-numbered channels are driven, then the resultant pressure pulses applied to the even-numbered channels can be no more than xc2xd of the magnitude of those applied to the intended channels. Again, the printing apparatus is arranged so that such xc2xd magnitude pulses do not cause ink ejection. Conversely, in the second cycle, the dividing walls to either side of channels numbered 2, 6, 10, . . . , for example, art driven (if ink is to be ejected from them) with the first pulsed signal, and the dividing walls to either side of channels 4, 8, 12, . . . are driven (if ink is to be ejected from them) with the second pulsed signal which is xcfx80 radians out-of-phase with the first pulsed signal.
A problem with the driving schemes described above is that the requirement for ⅔ or xc2xd of the channels not to be printing at any time means that the speed of printing is not as high as might be desired.
In accordance with the first aspect of the invention, the method of operating such an apparatus comprises the steps of: developing for each channel a respective pulsed electrical signal in dependence upon whether ink is to be ejected from that channel, the signals for adjacent pairs of the channels temporally overlapping each other and not being in phase with each other; and applying to each dividing wall a combination of the signals developed for the respective pair of the channels.
It might be expected that with such a method of operation, unwanted ink ejection will occur. However, as will be apparent from the following description, on the whole, the method enables no more than xc2xd size pulses to be applied to channels which are not intended to eject ink. When a particular channel is intended to eject, and one of its adjacent channels is also, but the other adjacent channel is not, then the middle channel may be subjected to pulses which are slightly greater than normal, slightly distorted and/or slightly phase-shifted compared with normal, but these effects can be arranged so that they are not significant. The method of this aspect of the invention enables ink to be ejected, when necessary, from all of the outlets at the same time, and thus the printing speed can be high.
Preferably, the channels are arranged in groups each containing a number X(Xxe2x89xa73, for example four) of the channels, the signals developed for adjacent pairs of the channels being generally 2xcfx80n/X radians out of phase with each other, where n is an integer not equal to X. Thus, there is a substantially constant phase shift between one channel and the next. Preferably n and X have no common factors.
Preferably, for each dividing wall, the applying step comprises the steps of: applying the signal developed for the channel to one side of that dividing wall to an electrode on that side of the dividing wall; and applying the signal developed for the channel to the other side of that dividing wall to an electrode on that other side of the dividing wall.
Preferably, lines of pixels of the ink are deposited onto the recording medium in respective cycles; and, in each cycle, those of the channels which are to eject ink in that cycle begin ejection at the beginning of the cycle and continue ejection until part-way through, or the end of, the cycle. The effect of this is that, if there is a slight phase-shift when, as described above, a particular channel is ejecting, and one of its adjacent channels is also, but the other adjacent channel is not, two such phase shifts in opposite directions will not occur for a single channel in succession.
The signals developed for each channel may be generally square-wave signals or generally sinusoidal.
In accordance with a second aspect of the present invention, there is provided a drop-on-demand multi-tone printing apparatus, comprising: a series of channels arranged to receive ink from a source, each channel having a respective ink outlet, each adjacent pair of the channels being separated by a respective dividing wall, and each dividing wall being movable in response to a pulsed electrical signals to apply pressure pulses to the ink in the respective pair of the channels, whereby ink can be ejected from the outlets and deposited onto a recording medium; means for developing for each channel a respective pulsed electrical signal in dependence upon whether ink is to be ejected from that channel, the signals for adjacent pairs of the channels temporally overlapping each other and not being in phase with each other; and means for applying to each dividing wall a combination of the signals developed for the respective pair of the channels.
The printing apparatus may be provided with various features so as to perform the preferred steps of the method described above.